A quantum well is, in its simplest form, a double heterostructure, with a layer of low band-gap material sandwiched between two layers of higher band-gap material. The layer of low band-gap material has a thickness, typically of 100 Angstroms or less thus providing a region of confinement for charge carriers in which the physical dimensions are such that quantum effects are significant.
By altering an electric field applied across a quantum well it is possible to move the optical absorption edge. Because a quantum well has a thickness of typically 100 Angstroms or less, the modulation depth which can be achieved with a single quantum well device is small. (The interactions for light incident normally on a single quantum well is extremely low.) Therefore devices usually, but not always, include stacks of 20 or more quantum wells in order to achieve a useful modulation depth. Such devices may be used as modulators and may be operated in reflection, transmission, or waveguide mode. In transmission mode light is incident normally on the device surface and passes through the device. In reflection mode devices are arranged so that light at an appropriate angle of incidence is reflected from or within the device. In waveguide mode light propagates parallel to the planes of the quantum well layers.
Typically, individual modulators made from multiple quantum wells may have optically active regions with diameters in the range 50 to 250 microns (micro-meters) or more, and may be constructed in the form of an array of devices. Ideally, such modulators or attenuators should on application of an electric field introduce an attenuation change of the order of 10 dB or more, although many devices will give adequate performance with less than 10 dB. By using pin diodes made from multiple quantum well structures, a 3 dB increase in absorption has been reported in planar GaInAs/InP modulators by applying a reverse bias of 30 volts (Electronic Letters Vol 24, pages 1583-1584 (1988)). These devices had an active region of diameter 60 microns. 4.times.4 arrays of such devices have been used to demonstrate transmission through a 100 M/bit per second -16 channel parallel interconnect system.
Although multiple quantum well modulators have been shown to operate at high speed, their use in the implementation of two dimensional arrays in systems have been limited by the combination low contrast ratio and high operating voltage. (Fabrication of multiple quantum well guides is a route to one dimensional arrays with improved characteristics--IKOGAKA et Al IEEE Photonics Tech Lett, Vol 1 (5), Pages 100-101 1989).
A known method of enhancing modulation for low applied voltage is to replace thin InGaAs wells (typically 60 Angstroms in width for operation at a wavelength of 1.5 microns) with coupled quantum wells. The separation of two sub-wells of thickness 30 Angstroms, by a thin (20 Angstrom) InP barrier, renders the quantum well wave functions more sensitive to an applied field.